Wireless communication systems are widely spread all over the world to provide various types of communication services such as voice or data. In general, the wireless communication system is a multiple access system capable of supporting communication with multi-users by sharing available radio resources. Examples of the radio resource include time, frequency, code, transmit power, etc. Examples of the multiple access system include a time division multiple access (TDMA) system, a code division multiple access (CDMA) system, a frequency division multiple access (FDMA) system, an orthogonal frequency division multiple access (OFDMA) system, a single carrier frequency division multiple access (SC-FDMA) system, etc. The radio resource is a time in the TDMA system, a frequency in the FDMA system, a code in the CDMA system, and a subcarrier and a time in the OFDMA system. A wireless communication system is a system supporting bidirectional communication. The bidirectional communication can be performed by using a time division duplex (TDD) mode, a frequency division duplex (FDD) mode, a half-frequency division duplex (H-FDD) mode, etc. The TDD mode uses a time resource to identify uplink transmission and downlink transmission. The FDD mode uses a frequency resource to identify uplink transmission and downlink transmission. The H-FDD mode uses a combination of a time resource and a frequency resource to identify uplink transmission and downlink transmission.
The wireless communication system includes a base station (BS) providing a service to a specific region (i.e., a cell). According to a characteristic of a wireless transmission technology, changes in a wireless environment have an effect on quality of signal transmitted. In particular, due to various factors in a surrounding environment, such as scatters, movement of a mobile station (MS), etc., a wireless channel changes over time. In addition, there is a restriction in terms of distance since reception power is rapidly decreased in proportion to a distance between wireless communication entities. Therefore, in general, the MS can communicate with the BS when the MS is located within the coverage of the BS. As such, due to several factors such as the scatters, a movement speed of the MS, a distance between transmission and reception, etc., a maximum transfer rate, a throughput of an intra-cell user, and a throughput of a whole cell are decreased between the BS and the MS. For example, when the MS is located in a cell boundary or when an obstacle such as a building exists between the MS and the BS, communication quality between the MS and the BS may not be satisfactory.
As an effort to overcome the aforementioned problem, several techniques are introduced to compensate for deterioration of signals transmitted between the BS and the MS, thereby obtaining a maximum transfer rate, throughput improvement, coverage expansion, etc. For this purpose, a wireless communication system may employ a relay station (RS). For example, a relay technique is one of main techniques for long term evolution (LTE)-advanced which is one of promising candidates of international mobile telecommunication (IMT)-advanced, i.e., a mobile communication system of a next generation (post-3rd generation). The RS is an apparatus for relaying a signal between the BS and the MS, and can extend the coverage of the BS and improve a cell throughput.